AsyncTask在Android中是很常用的异步线程，那么AsyncTask和Thread有什么区别呢？这里将从源码角度深入理解AsyncTask的设计和工作原理
这里的AsyncTask基于SDK-25
分析知识准备
首先我们来看一个生产者与消费者模型的例子
public class ThreadTest {
//产品
static class ProductObject{
public volatile static String value; //volatile线程操作变量可见
}
//生产者线程
static class Producer extends Thread{
Object lock;
public Producer(Object lock) {
this.lock = lock;
}
@Override
public void run() {
while(true){
synchronized (lock) {
if(ProductObject.value != null){
try {
lock.wait(); //产品还没有被消费，等待
} catch (InterruptedException e) {
e.printStackTrace();
}
}
ProductObject.value = "NO:"+System.currentTimeMillis();
System.out.println("生产产品："+ProductObject.value);
lock.notify(); //生产完成，通知消费者消费
}
}
}
}
//消费者线程
static class Consumer extends Thread{
Object lock;
public Consumer(Object lock) {
this.lock = lock;
}
@Override
public void run() {
while(true){
synchronized (lock) {
if(ProductObject.value == null){
try {
lock.wait(); //等待，阻塞
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("消费产品："+ProductObject.value);
ProductObject.value = null;
lock.notify(); //消费完成，通知生产者，继续生产
}
}
}
}
public static void main(String[] args) {
Object lock = new Object();
new Producer(lock).start();
new Consumer(lock).start();
}
}
上面的例子关键点在于两个，其一是volatile，使得线程间可见，第二个点在于互斥锁，这样就可以使得有商品的时候就要通知消费者消费，同时wait，那么消费者收到消息开始消费，消费完毕通知生产者继续生产，从而不断生产，这样比轮询方式更加节省资源
在了解完上面的例子以后，我们就可以着手分析AsyncTask的源代码了
首先，我们在AsyncTask首先看其构造方法
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
···
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
这里首先给WorkerRunnable和Future进行了初始化，那么为何要初始化这两个变量呢？
这里就要说到常用的两个方法了，doInBackground()，这个方法是在子线程里面完成的，另一个方法就是onPostExecute()，而这个方法是存在于主线程的，那么也就是说子线程执行完将执行的结果传递到了主线程中，实现了线程间的通信，那么最关键的问题来了，这个通信是怎么实现的呢？
通常在子线程中执行的任务，是没有返回结果的，例如Runnable的源代码如下，就没有返回结果
public interface Runnable {
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see java.lang.Thread#run()
*/
public abstract void run();
}
那么，要怎么才能得到返回值呢，这里首先想到的就是Callable接口，那么再看看Callable的源代码
@FunctionalInterface
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}
可以看到，这是一个泛型方法，是有返回值的，但是其本身确是不能直接执行的，需要借助其他类，接下来再看一看源代码中涉及到的Future接口
public interface Future<V> {
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
在Future类中，有好几个方法，而这些方法都是有返回值的，那么Runnable与Future和FutureTask有什么关系呢，产看源码便可得知，FutureTask实际上是实现了RunnableFuture接口
public class FutureTask<V> implements RunnableFuture<V>{
···
}
而RunnableFuture又继承了Runnable和Future
public interface RunnableFuture<V> extends Runnable, Future<V> {
void run();
}
那也就是说，FutureTask既可以在子线程中执行，也可以获得执行结果，下面使用一个例子来说明FutureTask
public class FutureTest {
public static void main(String[] args) {
Task work = new Task();
FutureTask<Integer> future = new FutureTask<Integer>(work){
@Override
protected void done() { //异步任务执行完成，回调
try {
System.out.println("done:" + get()); //get()获取异步任务的返回值，这是个阻塞方法
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
};
//线程池（使用了预定义的配置）
ExecutorService executor = Executors.newCachedThreadPool();
executor.execute(future);
}
//异步任务
static class Task implements Callable<Integer>{
@Override
public Integer call() throws Exception {//返回异步任务的执行结果
int i = 0;
for (; i < 10; i++) {
try {
System.out.println(Thread.currentThread().getName() + "_" + i);
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
return i;
}
}
}
上面的例子可以看出，在使用了Callable的时候，需要借助FutureTask来包装，然后使用Executor的execute()方法来执行，那么是怎么得到异步任务的返回值呢，在上面的例子中，我们可以看到，其返回值的获取是通过future.get()得到的，然而这个get()方法确是被阻塞的，只有在异步任务完成的时候才能获取到其结果，那我们怎么才能知道异步任务时候执行完毕呢，这里就可以实现FutureTask的done()方法，当异步任务执行完毕以后会回调这个方法，上述例子其实解释了AsyncTask的实现逻辑，call()方法是在子线程中完成，这也就是doInBackground()的实现，在主线程中获得结果，这是在onPostExecute()使用了get()方法，那也就是说AsyncTask就是通过这一套方法去实现的
从这里我们可以总结出FutureTask为异步任务提供了诸多便利性，包括
- 获取异步任务的返回值
- 监听异步任务的执行情况
- 取消异步任务
那么在AsyncTask中，WorkerRunnable又是啥呢，其实就是一个内部类，对Callable进行了封装
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
源代码分析
有了以上知识储备，我们就可以动手分析AsyncTask源代码了
拿到源代码，不同的人有不同的分析习惯，这里我按照我的习惯对源代码进行一次分析
构造方法分析
首先，因为我们分析源代码是为了更好的去使用，而使用的话，第一个关注的就应该是构造方法，回到之前的的构造方法，这里要开始对构造方法开始入手分析了
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
···
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
//设置线程调用
mTaskInvoked.set(true);
Result result = null;
try {
//设置线程优先级，其给定值为10
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//调用doInBackground()方法得到返回值
result = doInBackground(mParams);
//将当前线程中的Binder命令发送至kernel
Binder.flushPendingCommands();
} catch (Throwable tr) {
//发生异常则取消线程调用设置
mCancelled.set(true);
throw tr;
} finally {
//执行postResult()方法
postResult(result);
}
return result;
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
//执行postResultIfNotInvoked()方法
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
以上就是AsyncTask的构造方法了，在构造方法上有一句说明，这个构造方法必须在UI线程中创建，这一点很好理解，因为其有需要再主线程中执行的地方，后面会说到，那么这个构造方法干了什么事情呢，很简单，这里新建了两个对象，首先是WorkerRunnable，而这个WorkerRunnable则是实现自Callable接口，主要是要使用其call()方法，为了返回参数，并没有什么特别之处，再其内部则实现了call()方法，而其自生是无法执行的，需要找一个包装类，而这个包装类就是FutureTask，通过之前的分析，这里就不再多赘述关于FutureTask的东西了，这里实现了done()方法，也就是线程执行完毕调用的方法，简单点来说就是在call()方法中执行，在done()中获得执行的返回结果，上述涉及到一个内部类和三个自定义的方法，那么接下来我们看一看这个内部类和三个方法都干了啥
构造方法中出现的内部类
这里的WorkerRunnable，正如前面所说，这里除了实现Callable就啥也没干，还是个抽象方法，这里将实现放在了构造方法中
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
构造方法中调用的方法
首先是doInBackground()方法，前面讲到，这个方法在子线程中完成，那么这里的子线程是哪个呢，其实就是WorkerThread，这个方法是一个抽象方法，放在子线程中执行，其具体实现由调用者完成
@WorkerThread
protected abstract Result doInBackground(Params... params);
再看postResult()方法，这里获取了一个Handler，然后发送了一个消息，这里就是子线程能够通信主线程的地方了
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
那么到这里，我们关注的重点就来了，子线程是怎么告诉主线程的呢，要知道其中的原因，我们就需要去看看代码里面是怎么实现的
我们先看发送了什么消息，也就是AsyncTaskResult里面干了啥，查看代码发现，其就是做了参数传递的任务
@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
那么接下来的重点就是getHandler()方法了，这里拿到AsyncTask.class就上了锁了，这也很好理解，不上锁其他线程走到这里会产生安全隐患，然后返回sHandler，那再继续看看InternalHandler又是个什么吧
private static InternalHandler sHandler;
···
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
这里就是了，在构造方法里面super(Looper.getMainLooper())，也就说明了这个方法是在主线程中执行的，在主线程中对Message进行处理，这里又涉及到两个方法，一个是finish()，还有一个是onProgressUpdate()，那么好吧，再去看看这两个方法在干啥
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
···
private static class InternalHandler extends Handler {
public InternalHandler() {
super(Looper.getMainLooper());
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
首先是又调了isCancelled()，判断是否取消，前面构造函数的时候见过这个，这是在异常发生的时候才设置为true的，那么如果不发生异常，这里应该就是为false的，但在找源代码时发现，另一个方法也对这个参数进行了设置，那就是cancel()，所以在不发生异常和取消的时候应该是为true的，接下来是onCancelled()方法，这里是不做任何操作的，这也是主线程中的方法，还有就是onPostExecute()方法，然后会设置状态，其默认状态是PENDING
private volatile Status mStatus = Status.PENDING;
···
public enum Status {
PENDING,
RUNNING,
FINISHED,
}
···
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
}
@SuppressWarnings({"UnusedParameters"})
@MainThread
protected void onCancelled(Result result) {
onCancelled();
}
@MainThread
protected void onCancelled() {
}
public final boolean isCancelled() {
return mCancelled.get();
}
···
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
然后是onProgressUpdate()方法，那么这里做了啥呢，嗯~啥也没有，交给调用者在继承时可以使用
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress... values) {
}
来看构造方法中涉及到的最后一个方法postResultIfNotInvoked()，这个方法又干了啥了，首先获得了mTaskInvoked的状态，整个AsyncTask只有构造方法处设置了这个值，然后判断是否执行postResult()方法
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
至此构造方法分析完成，可以看到在构造方法中，其主要做的工作最主要的就是搭建好了子线程和主线程沟通的桥梁
执行入口分析
在新建AsyncTask对象以后，要执行的话，需要使用execute()去开始执行
那么我们就从这里入手，看看其具体是怎么工作的，可以看到无论是构造方法还是启动方法，都是需要在主线程中完成的，在execute()中，做了些什么呢，在这之前我们先看看传递的sDefaultExecutor是啥
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
这其实是一个线程池，任务调度的线程池，可以看到SerialExecutor实际上是实现了Executor接口，其作用就是将任务添加到双向队列，然后不断地取出执行取出执行，那么THREAD_POOL_EXECUTOR也应该是一个线程池，那这又是啥呢，去看一看这个玩意儿就是到了
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
···
private static class SerialExecutor implements Executor {
//定义了一个双向队列，用来存储线程
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
//向队列中添加线程
mTasks.offer(new Runnable() {
public void run() {
try {
//线程运行
r.run();
} finally {
//执行scheduleNext()方法
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
//从队列中取出线程并执行
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
下列代码就是初始化了线程池的参数，指定了线程数量
//获得可用CPU数量
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
//设置核心线程池数量其范围[2,4]，无论是否使用都存在
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
//设置最大线程数量
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
//设置闲置回收时间，也就是说线程在这个时间内没有活动的话，会被回收
private static final int KEEP_ALIVE_SECONDS = 30;
//设置线程工厂，通过这个创建线程
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
//创建线程安全的线程个数计数器
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
//设置任务队列大小
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
//设置线程池
public static final Executor THREAD_POOL_EXECUTOR;
//初始化线程池
static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
//打开核心线程池的超时时间
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}
所以在执行scheduleNext()的时候，会将THREAD_POOL_EXECUTOR中设置好的线程全部取出来，用来执行后面的任务，其执行的任务就是execute()方法所指定的任务，在executeOnExecutor()方法中，由于前面初始化完成，这里的状态应该是PENDING，之后还设置了mWorker的参数，然后会执行线程池的方法，然后据开始执行任务了，前面没有涉及到的方法还有一个，那我们接下来看看
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
onPreExecute()，这个方法由调用者在继承时候能够使用
@MainThread
protected void onPreExecute() {
}
至此，AsyncTask的执行方法也分析完了，那么我们接下来看看还有什么方法没有涉及到，没有涉及到的方法都是public属性和方法
AsyncTask的公共方法
//这个方法设置为public，那么就意味着我们可以自定义线程池
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
//这个方法意味着我们可以获得其状态，配合枚举值使用
public enum Status {
PENDING,
RUNNING,
FINISHED,
}
···
public final Status getStatus() {
return mStatus;
}
//查看是非被取消
public final boolean isCancelled() {
return mCancelled.get();
}
//取消异步任务
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
//获取返回结果，注意：这个方法是阻塞式的
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
//同上
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
//自定义的线程池可以从这个方法启动
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
//使用默认线程池启动异步任务
@MainThread
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
总结
AsyncTask的实例化过程，其本质上就是实例化了一个FutureTask
其执行过程Executor.execute(mFuture) -> SerialExecutor.mTasks(队列) -> (线程池)THREAD_POOL_EXECUTOR.execute
线程池中的所有线程，为了执行异步任务
如果当前线程池中的数量小于corePoolSize，创建并添加的任务
如果当前线程池中的数量等于corePoolSize，缓冲队列workQueue未满，那么任务被放入缓冲队列、等待任务调度执行
如果当前线程池中的数量大于corePoolSize，缓冲队列workQueue已满，并且线程池中的数量小于maximumPoolSize，新提交任务会创建新线程执行任务
如果当前线程池中的数量大于corePoolSize，缓冲队列workQueue已满，并且线程池中的数量等于maximumPoolSize，新提交任务由Handler处理
当线程池中的线程大于corePoolSize时，多余线程空闲时间超过keepAliveTime时，会关闭这部分线程
线程池在添加时候是串行的，在执行任务的时候是并行的
附录(源代码)
package android.os;
import android.annotation.MainThread;
import android.annotation.WorkerThread;
import java.util.ArrayDeque;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
/**
* <p>AsyncTask enables proper and easy use of the UI thread. This class allows you
* to perform background operations and publish results on the UI thread without
* having to manipulate threads and/or handlers.</p>
*
* <p>AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler}
* and does not constitute a generic threading framework. AsyncTasks should ideally be
* used for short operations (a few seconds at the most.) If you need to keep threads
* running for long periods of time, it is highly recommended you use the various APIs
* provided by the <code>java.util.concurrent</code> package such as {@link Executor},
* {@link ThreadPoolExecutor} and {@link FutureTask}.</p>
*
* <p>An asynchronous task is defined by a computation that runs on a background thread and
* whose result is published on the UI thread. An asynchronous task is defined by 3 generic
* types, called <code>Params</code>, <code>Progress</code> and <code>Result</code>,
* and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>,
* <code>onProgressUpdate</code> and <code>onPostExecute</code>.</p>
*
* <div class="special reference">
* <h3>Developer Guides</h3>
* <p>For more information about using tasks and threads, read the
* <a href="{@docRoot}guide/components/processes-and-threads.html">Processes and
* Threads</a> developer guide.</p>
* </div>
*
* <h2>Usage</h2>
* <p>AsyncTask must be subclassed to be used. The subclass will override at least
* one method ({@link #doInBackground}), and most often will override a
* second one ({@link #onPostExecute}.)</p>
*
* <p>Here is an example of subclassing:</p>
* <pre class="prettyprint">
* private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
* protected Long doInBackground(URL... urls) {
* int count = urls.length;
* long totalSize = 0;
* for (int i = 0; i < count; i++) {
* totalSize += Downloader.downloadFile(urls[i]);
* publishProgress((int) ((i / (float) count) * 100));
* // Escape early if cancel() is called
* if (isCancelled()) break;
* }
* return totalSize;
* }
*
* protected void onProgressUpdate(Integer... progress) {
* setProgressPercent(progress[0]);
* }
*
* protected void onPostExecute(Long result) {
* showDialog("Downloaded " + result + " bytes");
* }
* }
* </pre>
*
* <p>Once created, a task is executed very simply:</p>
* <pre class="prettyprint">
* new DownloadFilesTask().execute(url1, url2, url3);
* </pre>
*
* <h2>AsyncTask's generic types</h2>
* <p>The three types used by an asynchronous task are the following:</p>
* <ol>
* <li><code>Params</code>, the type of the parameters sent to the task upon
* execution.</li>
* <li><code>Progress</code>, the type of the progress units published during
* the background computation.</li>
* <li><code>Result</code>, the type of the result of the background
* computation.</li>
* </ol>
* <p>Not all types are always used by an asynchronous task. To mark a type as unused,
* simply use the type {@link Void}:</p>
* <pre>
* private class MyTask extends AsyncTask<Void, Void, Void> { ... }
* </pre>
*
* <h2>The 4 steps</h2>
* <p>When an asynchronous task is executed, the task goes through 4 steps:</p>
* <ol>
* <li>{@link #onPreExecute()}, invoked on the UI thread before the task
* is executed. This step is normally used to setup the task, for instance by
* showing a progress bar in the user interface.</li>
* <li>{@link #doInBackground}, invoked on the background thread
* immediately after {@link #onPreExecute()} finishes executing. This step is used
* to perform background computation that can take a long time. The parameters
* of the asynchronous task are passed to this step. The result of the computation must
* be returned by this step and will be passed back to the last step. This step
* can also use {@link #publishProgress} to publish one or more units
* of progress. These values are published on the UI thread, in the
* {@link #onProgressUpdate} step.</li>
* <li>{@link #onProgressUpdate}, invoked on the UI thread after a
* call to {@link #publishProgress}. The timing of the execution is
* undefined. This method is used to display any form of progress in the user
* interface while the background computation is still executing. For instance,
* it can be used to animate a progress bar or show logs in a text field.</li>
* <li>{@link #onPostExecute}, invoked on the UI thread after the background
* computation finishes. The result of the background computation is passed to
* this step as a parameter.</li>
* </ol>
*
* <h2>Cancelling a task</h2>
* <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking
* this method will cause subsequent calls to {@link #isCancelled()} to return true.
* After invoking this method, {@link #onCancelled(Object)}, instead of
* {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])}
* returns. To ensure that a task is cancelled as quickly as possible, you should always
* check the return value of {@link #isCancelled()} periodically from
* {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p>
*
* <h2>Threading rules</h2>
* <p>There are a few threading rules that must be followed for this class to
* work properly:</p>
* <ul>
* <li>The AsyncTask class must be loaded on the UI thread. This is done
* automatically as of {@link android.os.Build.VERSION_CODES#JELLY_BEAN}.</li>
* <li>The task instance must be created on the UI thread.</li>
* <li>{@link #execute} must be invoked on the UI thread.</li>
* <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute},
* {@link #doInBackground}, {@link #onProgressUpdate} manually.</li>
* <li>The task can be executed only once (an exception will be thrown if
* a second execution is attempted.)</li>
* </ul>
*
* <h2>Memory observability</h2>
* <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following
* operations are safe without explicit synchronizations.</p>
* <ul>
* <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them
* in {@link #doInBackground}.
* <li>Set member fields in {@link #doInBackground}, and refer to them in
* {@link #onProgressUpdate} and {@link #onPostExecute}.
* </ul>
*
* <h2>Order of execution</h2>
* <p>When first introduced, AsyncTasks were executed serially on a single background
* thread. Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
* to a pool of threads allowing multiple tasks to operate in parallel. Starting with
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single
* thread to avoid common application errors caused by parallel execution.</p>
* <p>If you truly want parallel execution, you can invoke
* {@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with
* {@link #THREAD_POOL_EXECUTOR}.</p>
*/
public abstract class AsyncTask<Params, Progress, Result> {
private static final String LOG_TAG = "AsyncTask";
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
// We want at least 2 threads and at most 4 threads in the core pool,
// preferring to have 1 less than the CPU count to avoid saturating
// the CPU with background work
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE_SECONDS = 30;
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
/**
* An {@link Executor} that can be used to execute tasks in parallel.
*/
public static final Executor THREAD_POOL_EXECUTOR;
static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}
/**
* An {@link Executor} that executes tasks one at a time in serial
* order. This serialization is global to a particular process.
*/
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
private static InternalHandler sHandler;
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private volatile Status mStatus = Status.PENDING;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
private static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
/**
* Indicates the current status of the task. Each status will be set only once
* during the lifetime of a task.
*/
public enum Status {
/**
* Indicates that the task has not been executed yet.
*/
PENDING,
/**
* Indicates that the task is running.
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
*/
FINISHED,
}
private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}
/** @hide */
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*/
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
/**
* Returns the current status of this task.
*
* @return The current status.
*/
public final Status getStatus() {
return mStatus;
}
/**
* Override this method to perform a computation on a background thread. The
* specified parameters are the parameters passed to {@link #execute}
* by the caller of this task.
*
* This method can call {@link #publishProgress} to publish updates
* on the UI thread.
*
* @param params The parameters of the task.
*
* @return A result, defined by the subclass of this task.
*
* @see #onPreExecute()
* @see #onPostExecute
* @see #publishProgress
*/
@WorkerThread
protected abstract Result doInBackground(Params... params);
/**
* Runs on the UI thread before {@link #doInBackground}.
*
* @see #onPostExecute
* @see #doInBackground
*/
@MainThread
protected void onPreExecute() {
}
/**
* <p>Runs on the UI thread after {@link #doInBackground}. The
* specified result is the value returned by {@link #doInBackground}.</p>
*
* <p>This method won't be invoked if the task was cancelled.</p>
*
* @param result The result of the operation computed by {@link #doInBackground}.
*
* @see #onPreExecute
* @see #doInBackground
* @see #onCancelled(Object)
*/
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
}
/**
* Runs on the UI thread after {@link #publishProgress} is invoked.
* The specified values are the values passed to {@link #publishProgress}.
*
* @param values The values indicating progress.
*
* @see #publishProgress
* @see #doInBackground
*/
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress... values) {
}
/**
* <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
* {@link #doInBackground(Object[])} has finished.</p>
*
* <p>The default implementation simply invokes {@link #onCancelled()} and
* ignores the result. If you write your own implementation, do not call
* <code>super.onCancelled(result)</code>.</p>
*
* @param result The result, if any, computed in
* {@link #doInBackground(Object[])}, can be null
*
* @see #cancel(boolean)
* @see #isCancelled()
*/
@SuppressWarnings({"UnusedParameters"})
@MainThread
protected void onCancelled(Result result) {
onCancelled();
}
/**
* <p>Applications should preferably override {@link #onCancelled(Object)}.
* This method is invoked by the default implementation of
* {@link #onCancelled(Object)}.</p>
*
* <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
* {@link #doInBackground(Object[])} has finished.</p>
*
* @see #onCancelled(Object)
* @see #cancel(boolean)
* @see #isCancelled()
*/
@MainThread
protected void onCancelled() {
}
/**
* Returns <tt>true</tt> if this task was cancelled before it completed
* normally. If you are calling {@link #cancel(boolean)} on the task,
* the value returned by this method should be checked periodically from
* {@link #doInBackground(Object[])} to end the task as soon as possible.
*
* @return <tt>true</tt> if task was cancelled before it completed
*
* @see #cancel(boolean)
*/
public final boolean isCancelled() {
return mCancelled.get();
}
/**
* <p>Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when <tt>cancel</tt> is called,
* this task should never run. If the task has already started,
* then the <tt>mayInterruptIfRunning</tt> parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.</p>
*
* <p>Calling this method will result in {@link #onCancelled(Object)} being
* invoked on the UI thread after {@link #doInBackground(Object[])}
* returns. Calling this method guarantees that {@link #onPostExecute(Object)}
* is never invoked. After invoking this method, you should check the
* value returned by {@link #isCancelled()} periodically from
* {@link #doInBackground(Object[])} to finish the task as early as
* possible.</p>
*
* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete.
*
* @return <tt>false</tt> if the task could not be cancelled,
* typically because it has already completed normally;
* <tt>true</tt> otherwise
*
* @see #isCancelled()
* @see #onCancelled(Object)
*/
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return The computed result.
*
* @throws CancellationException If the computation was cancelled.
* @throws ExecutionException If the computation threw an exception.
* @throws InterruptedException If the current thread was interrupted
* while waiting.
*/
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result.
*
* @param timeout Time to wait before cancelling the operation.
* @param unit The time unit for the timeout.
*
* @return The computed result.
*
* @throws CancellationException If the computation was cancelled.
* @throws ExecutionException If the computation threw an exception.
* @throws InterruptedException If the current thread was interrupted
* while waiting.
* @throws TimeoutException If the wait timed out.
*/
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
/**
* Executes the task with the specified parameters. The task returns
* itself (this) so that the caller can keep a reference to it.
*
* <p>Note: this function schedules the task on a queue for a single background
* thread or pool of threads depending on the platform version. When first
* introduced, AsyncTasks were executed serially on a single background thread.
* Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
* to a pool of threads allowing multiple tasks to operate in parallel. Starting
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being
* executed on a single thread to avoid common application errors caused
* by parallel execution. If you truly want parallel execution, you can use
* the {@link #executeOnExecutor} version of this method
* with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings
* on its use.
*
* <p>This method must be invoked on the UI thread.
*
* @param params The parameters of the task.
*
* @return This instance of AsyncTask.
*
* @throws IllegalStateException If {@link #getStatus()} returns either
* {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
*
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
* @see #execute(Runnable)
*/
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
/**
* Executes the task with the specified parameters. The task returns
* itself (this) so that the caller can keep a reference to it.
*
* <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to
* allow multiple tasks to run in parallel on a pool of threads managed by
* AsyncTask, however you can also use your own {@link Executor} for custom
* behavior.
*
* <p><em>Warning:</em> Allowing multiple tasks to run in parallel from
* a thread pool is generally <em>not</em> what one wants, because the order
* of their operation is not defined. For example, if these tasks are used
* to modify any state in common (such as writing a file due to a button click),
* there are no guarantees on the order of the modifications.
* Without careful work it is possible in rare cases for the newer version
* of the data to be over-written by an older one, leading to obscure data
* loss and stability issues. Such changes are best
* executed in serial; to guarantee such work is serialized regardless of
* platform version you can use this function with {@link #SERIAL_EXECUTOR}.
*
* <p>This method must be invoked on the UI thread.
*
* @param exec The executor to use. {@link #THREAD_POOL_EXECUTOR} is available as a
* convenient process-wide thread pool for tasks that are loosely coupled.
* @param params The parameters of the task.
*
* @return This instance of AsyncTask.
*
* @throws IllegalStateException If {@link #getStatus()} returns either
* {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
*
* @see #execute(Object[])
*/
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
/**
* Convenience version of {@link #execute(Object...)} for use with
* a simple Runnable object. See {@link #execute(Object[])} for more
* information on the order of execution.
*
* @see #execute(Object[])
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
*/
@MainThread
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
/**
* This method can be invoked from {@link #doInBackground} to
* publish updates on the UI thread while the background computation is
* still running. Each call to this method will trigger the execution of
* {@link #onProgressUpdate} on the UI thread.
*
* {@link #onProgressUpdate} will not be called if the task has been
* canceled.
*
* @param values The progress values to update the UI with.
*
* @see #onProgressUpdate
* @see #doInBackground
*/
@WorkerThread
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
private static class InternalHandler extends Handler {
public InternalHandler() {
super(Looper.getMainLooper());
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
}